1. Field of the Invention
The present invention relates to the regulation of the composition of an atmosphere in an enclosure, for example a furnace or oven.
By way of illustration, mention may be made here of the example of regulating the oxygen level in nitrogen-based atmospheres used for heat treatment applications carried out on metal or ceramic articles, or alternatively for soldering operations.
2. Description of Related Art
Selecting an atmosphere of given composition, for carrying out a certain operation in the enclosure (for example a heat treatment operation), most often entails definition of at least one maximum level of a given element in this atmosphere.
By way of example, selecting a nitrogen-based atmosphere for a specific process very often entails definition of a maximum oxygen level in this atmosphere. A case which may be considered by way of illustration is that of mounting electronic components on a printed circuit using a soldering paste, in so-called reflow ovens, for which the use of certain soldering paste formulations gives rise to recommendations in terms of the maximum tolerable oxygen level in the soldering atmosphere.
It will therefore be understood that the required nitrogen-based atmosphere will always be obtained from a nitrogen source in which the oxygen level is less than the maximum acceptable level in the process atmosphere, in view of the conventional precautions taken in terms of injection and regulation (the flow rates which are used, the geographical distribution of the injections, the leak tightness of the enclosure, etc.).
It is further known that, for a given gas source and a given configuration of the enclosure and the setting of the operating parameters of this enclosure, the level of oxygen (and more generally of the element in question) in the process atmosphere will often vary, in particular on the basis of the production rate of the processing enclosure, or alternatively because of external conditions such as the creation of air currents in the workshop where the enclosure in question is located.
Even so, a clear desire is increasingly being felt by users to gain better control over their processes, in order to achieve better reproducibility in the quality of the articles delivered from the enclosure.
Process control of this type necessarily involves controlling the atmosphere which is used inside the enclosure, it clearly being insufficient just to control the quality of the gas sources which are supplied.
With regard to the example of nitrogen-based atmospheres having a controlled oxygen level, existing solutions for providing such atmosphere control can essentially be put into two categories:
Solution No. 1: Excess quality
According to this first solution, use is made of regulation (flow rate and distribution of the injections) and a nitrogen source with given oxygen purity, which are such that, for any variations or disturbances which occur in the process, the residual oxygen level measured in the process atmosphere is always less than the maximum acceptable level.
By way of example, a high flow rate of cryogenically produced nitrogen could be used here, even though the user process could readily tolerate a few thousands, or even a few tens of thousands of ppm of residual oxygen.
Solution No. 2: Atmosphere control by flow rate regulation
According to this second solution, use is made of a degree of regulation (flow rate and distribution of the gas injections) and a source of nitrogen with given oxygen purity, which are limited enough for there actually to be variations or disturbances in the operation of the enclosure, which may cause the maximum acceptable residual oxygen level to be exceeded in the process atmosphere.
The gas source is then used in combination with a regulation system which, when the residual oxygen level measured in the atmosphere of the enclosure exceeds a given upper threshold, makes it possible to increase the flow rate of gas into the enclosure (which is then in an "excess atmosphere" situation), the flow rate of gas injected into the enclosure being reduced again only when the oxygen level measured in the process atmosphere has returned below the threshold.
It can therefore be seen that, on the one hand, the first solution, like all solutions using excess quality, is needlessly expensive, and yet without completely eliminating the risk of an unexpected disturbance leading to the maximum acceptable residual oxygen level being exceeded in the process atmosphere.
For its part, the second solution does indeed allow the purity of the gas source and/or the flow rate used to be adapted better to the actual process requirements, but may nevertheless in the long term lead to a relatively high total gas consumption. It may further be noted that the fact that modification the flow rates used in the enclosure in this way when the oxygen threshold is exceeded, inevitably entails instability phenomena in the process:
the induced flow rate variation is, on the one hand, a source of turbulence which promotes air being taken into the enclosure; PA1 such a variation in the flow rate used further leads to a variation in the (convective) heat transfer produced inside the enclosure when the enclosure is the site of such a heat treatment operation.
Mention may further be made of the fact that even though, in most cases, the selection of a controlled atmosphere for a process entails definition of a maximum level of a given element in this atmosphere, there are cases in which it is necessary to define not only a maximum level but also a minimum level, which is equivalent to defining a tolerable range for the element in question in the atmosphere. Mention may be made here by way of illustration of the case of soldering electronic components onto a printed circuit using a soldering paste, for which the observation, in the case of soldering under nitrogen as a replacement for air, of a number of new defects such as the tombstoning effect, also referred to as the Manhattan effect (an electronic component lifting off during the soldering phase, leading to opening of the electrical circuit), has led to the adoption of tolerable intervals for the oxygen level in the nitrogen, in order to avoid the presence of these defects. This application example will be dealt with in more detail below in the scope of the present application.